Mycoplasma bovis is a pathogenic prokaryote belonging to a class of organisms that is intermediate in size between a bacteria and virus. These mycoplasmas are the smallest of the free-living microorganisms. They are characterized by the lack of a cell wall and are enveloped with only a cell membrane, which allows for varying morphological shapes and unique growth requirements.
Mycoplasmas are known to cause infectious disease in most species of animals. In bovine species, Mycoplasma bovis is an opportunistic microorganism that causes infectious disease that is of significant economic importance to the livestock industry. Mycoplasma bovis isolation in a diseased bovine can be the result of its role as either a primary or secondary causative etiological disease agent. Clinical disease and losses associated with infections caused by Mycoplasma bovis in beef and dairy cattle include: contagious mastitis, respiratory pneumonia, joint infections (arthritic conditions), keratoconjunctivitis, and middle ear infections. Even though several species of mycoplasmas have been isolated in cattle, by far the most prevalent is Mycoplasma bovis. For mastitis infections, the prevalence of M. bovis is reported to be 70% or more.
Diseases caused by mycoplasmas are often resistant to antimicrobial therapy, leaving no effective means of treatment. Consequently, the only effective control method is to cull animals from a herd. This has enormous economic implications in the dairy industry where losses are measured by the value of the culled animals as well as the impact on both milk quality and quantity due to clinical and subclinical infections. Mycoplasma infections resulting in bovine mastitis are increasing in prevalence and geographical distribution. In the United States, this higher prevalence is due to a larger and more intense cattle production industry in which herds are rapidly expanding, placing them at greater risk. Increased incidence of M. bovis infection and related infectious disease in dairy herds has been noted worldwide (Jasper, D E 1982, J. Amer. Vet. Med. Assn. 181:158-162).
Control of disease caused by mycoplasmas in swine and avian species has occurred in recent years as the result of successful vaccination programs using safe and efficacious products. The design and development of an effective commercial vaccine in the United States to control Mycoplasma bovis has not yet occurred, even though changes in cattle production methods and husbandry practices have resulted in a greater commercial need to control this agent from both an economic and food quality perspective. Although there have been numerous attempts to produce vaccine to protect against Mycoplasma bovis, the resulting experimental vaccines have been deemed unacceptable due to the lack of protection as well as unacceptable site reactions in vaccinated animals (Boothby, et al. 1986 Cornell Vet 76: 188-197; Boothby et al. 1987 Can J. Veterinary Research 51:121-125; Howard et al. 1987 Veterinary Record 121:372-376; Boothby, et al. 1988 Can J. Veterinary Research 52:355-359). Thus, there remains a need in the veterinary and animal health profession to provide a safe and effective vaccine to prevent infectious disease caused by Mycoplasma bovis with no unfavorable host animal reactions.